The telecommunications industry has been working with polarization adjusters, controllers, and scramblers for many years. Typical uses include: optimizing optical power transmission through polarization dependent components; detecting polarization dependence of a component (by monitoring optical power at the output of the component while methodically scanning all the polarization states at the input of the component); and curtailing polarization dependence of components and detectors (by scanning substantially all possible states of polarization and illuminating components and detectors at a rate faster than the signal sampling rate). Polarization controllers having endless control capabilities, which allow for reset free operation, are also in use in polarization mode dispersion compensators. Heismann teaches such an endless polarization controller in U.S. Pat. No. 5,212,743 issued May 18, 1993 incorporated herein by reference.
Controlling the state of Polarization (SOP) in optical fibers has long been an active research topic with many papers published in the last twenty years: T. Imai, K. Nosu, and H Yamaguchi, “Optical polarisation control utilising an optical heterodyne detection scheme”, Electron. Lett., vol. 21, pp. 52-53, January 1985; T. Okoshi, “Polarization-state control schemes for heterodyne or homodyne optical fiber communications”, J. Lightwave Technol., vol. 3, pp. 1232-1237, December 1985; R. Noé, H. Heidrich, and D. Hoffmann, “Endless Polarization Control Systems for Coherent Optics”, J. Lightwave Technol., vol. 6, pp. 1199-1208, July 1988; W. H. J. Aarts and G. D. Khoe, “New Endless Polarization Control Method Using Three Fiber Squeezers”, J. Lightwave Technol., vol. 7, pp. 1033-1043, July 1989; F. Heismann, “Integrated-Optic Polarization Transformer for Reset-Free Endless Polarization Control”, IEEE J. Quantum Electron., vol. 25, pp. 1898-1906, August 1989; N. G. Walker and G. R. Walker,” Polarization Control for Coherent Communications”, J. Lightwave Technol., vol. 8, pp. 438-458, March 1990; and S. H. Rumbaugh, M. D. Jones, and L. W. Casperson, “Polarization Control for Coherent Systems Using Nematic Liquid Crystals”, J. Lightwave Technol., vol. 8, pp. 459-465, March 1990, all of which are incorporated herein by reference. Those articles dealt with devices for polarization control, e.g. fiber squeezers, liquid crystals, rotating waveplates, or waveguide devices including electro-optical materials such as LiNbO3, and with algorithms for polarization control. The principal objective of these algorithms was to provide a polarization controller (PC) with unlimited transformation ranges. This property of being able to generate an arbitrary sequence of continuously varying polarization states is referred to as endlessness.
The transducers used as polarization control devices fall into two categories: variable retardation plates (VRP) and rotatable retarders. VRPs introduce an adjustable retardance between two orthogonal polarization components of the optic field; they are generally electrically adjustable retarders. Rotatable waveplates are linear retarders such as quarter wave plates (QWP) and half wave plates (HWP) mounted in rotary stages so their optical axis can be rotated. Devices based on LiNbO3 waveguides can fall into either category with three or more electrodes required to rotate the axis of retardation. The main difference between these two classes of device is that VRPs have a finite retardance range and rotatable waveplates can be endlessly rotated. The retardance range of a VRP is limited by the voltage range of its power supply and the device physics. VRPs such as liquid crystal cells (LC) have a maximum and minimum retardance and electro-optical modulators have practical limits to the maximum voltage. In contrast the optical axis of a waveplate can be endlessly rotated and the orientation angle can be increased or decreased without bound.
PCs based on rotatable waveplates can provide endless polarization transformations. The VRP-based PCs are generally more complex with more elements and elaborate computer-controlled drive algorithms. The lack of suitable low-cost, low-insertion-loss PCs has slowed the development of optical modules for polarization mode dispersion compensation (PMDC). LiNbO3-based PCs are quite expensive for such an application, whereas VRP-based PCs require driving algorithms that are complex for an efficient implementation. The cost and insertion loss of VRP-based PCs are more favourable than those related to LiNbO3-based PCs thereby providing motivation to explore VRP-based PC designs.
More specifically, VRPs are birefringent elements whose birefringence is varied by an externally applied voltage. The effect VRPs have on the SOP, represented as a point on the surface of a Poincaré sphere (M. Born and E. Wolf, Principles of Optics, 6th ed, Cambridge, U.K.: University Press, sixth edition, 1980, pp. 23-36, incorporated herein by reference), can be modeled by a rotation about an axis which is the fast axis of the element (D. S. Kliger, J. W. Lewis, and C. E. Randall, Polarized Light in Optics and Spectroscopy, San Diego, Calif.: Academic Press, 1997, pp. 103-152, incorporated herein by reference). For linearly birefringent VRPs the rotation axis lies on the equator of the sphere and the rotation angle equals the retardance of the device.